Each spring and summer, we see some cases of damping-off and root rot on bedding plants. These are always more likely to occur in wet areas and early in the season when tissues are more tender. There are a few steps you can take to help prevent some of these disease problems.
Plants that are stunted, low in vigor, slow-growing, or prone to wilting easily on a warm day may be infected with a root rot. Keep in mind that bedding plants grow slowly in cool temperatures. Root rots may also cause the foliage to turn yellow to brown and drop prematurely, usually starting with the older leaves and moving up the plant. The severity of the root rot depends on the fungal pathogen, the susceptibility of the host plant, and the soil and moisture conditions. In fact, dry conditions following infection by a root rot pathogen cause a more rapid decline of plants. Plants are not able to absorb water due to the inadequate root system. Drought accentuates this problem.
If a root rot is suspected, the plant should be carefully removed from the soil, placed in a bucket of water, and roots gently washed of soil so that they can be examined for indications of rotting. If roots are washed too vigorously, the rotted tissue is washed off, often leaving a white root interior that appears healthy. Close examination shows that such roots are much thinner than healthy white roots. Wash the roots by gently moving the plant up and down in a bucket of water until soil is removed. A healthy plant has numerous white roots that appear fibrous. It even has visible white root hairs. Roots of a diseased plant show various degrees of water-soaking and usually are some shade of brown or black. The discolored roots are often soft and mushy, while healthy roots are firm.
There are many root rot pathogens, but the major root rot fungi encountered in Illinois landscapes are Rhizoctonia, Fusarium, Pythium, and Phytophthora. In a very simplified scheme, we can group the first two fungi, those causing a dry rot, often with a reddish pink cast to affected roots, as either Rhizoctonia or Fusarium. Pythium and Phytophthora can be grouped as the types causing a soft, brown-to-black rot of roots.
Control of root rots should be aimed at prevention. Use only healthy transplants. This advice may sound silly, but don’t try to save money by purchasing weak plants. They may be diseased, and you certainly won’t save in the end. Proper site preparation to provide good water drainage away from roots is imperative. Pythium and Phytophthora are particular problems on wet sites. Dig the soil in the entire planting bed to a depth of about 10 inches, and work in organic matter if drainage needs to be improved. Use a balanced fertilizer if desired, but keep rates low on new transplants. Rotate plantings in the garden every 2 or 3 years with unrelated plants to help prevent the buildup of pathogens in one area. This practice is extremely helpful in preventing Fusarium and Rhizoctonia. Remove crop residue at the end of the season to help reduce pathogen survival.
Even if all the above practices are followed, root rot may still occur. Fungicides are available to control the major groups of fungi discussed here. The fungicides protect plant stems and roots not yet affected. Their use seems most significant in cases where a root rot is discovered in a flower bed and the goal is to preserve remaining healthy plants to the end of the season. Fungicide options are too numerous to list here. Specific chemicals are listed by host crop in the 2001 Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide (formerly Illinois Homeowners’ Guide to Pest Management). Consult Report on Plant Disease no. 615, “Damping-off and Root Rots of House Plants and Garden Flowers” for more details on root rots.